[mikeash]

The SSME also has to run from sea level all the way into space, whereas the Merlin is able to have one design optimized for sea level and a second design optimized for space.

The difference in efficiency is striking. The SSME's specific impulse (the closest equivalent to MPG in a car) is 452 in vacuum and 366 at sea level. Merlin's is 311/282 for the sea level version, and 348 for the vacuum version.

Of course, this is not a criticism of Merlin, just a comparison. By being less efficient, they're able to optimize for other stuff like cheapness and robustness.